This invention is directed to novel sulfur-modified chloroprene polymers, which have excellent mechanical properties and processability.
Sulfur-modified chloroprene polymers are well known and are available commercially under the trade designation Neoprene G. They are made by polymerization of chloroprene or copolymerization of chloroprene with another unsaturated organic monomer in the presence of elemental sulfur. The polymer, as made, contains sulfur atoms in its chain as well as sulfur bridges between different chains, so that it can be properly described as a highly crosslinked chloroprene/sulfur copolymer or chloroprene/organic monomer/sulfur copolymer. Because of its crosslinked structure, this polymer cannot be processed directly but must first be broken down to lower molecular weight. This step, known as peptization, is normally accomplished by treating polymer latex with a tetraalkylthiuram disulfide, sometimes in combination with a sodium dialkyl dithiocarbamate. Preparation and peptization of sulfur-modified chloroprene polymers is described, for example, in the following U.S. Pat. Nos. 2,234,215 (Youker); 3,595,847 (Mayer-Mader); and 3,920,623 (Khan).
In the commercially available G-type neoprenes, the amount of sulfur usually is in the upper portion of the 0.2-0.6 phr range. This introduces a rather large number of sulfur bridges, but opening of those bridges in the peptization process can give a product having fairly low molecular weight. Usually, it is desired to produce a polymer having a molecular weight above the theoretical minimum, and peptization is stopped before it is complete. The peptized polymer latex may have to be stored before the polymer can be isolated, preferably under conditions which would not lead to further molecular weight changes, such as further drop of molecular weight due to continuing peptization or increase of molecular weight due to crosslinking. Such undesirable changes affect both the processability and the mechanical properties of the polymer.
There is a need for a sulfur-modified chloroprene polymer which is resistant to viscosity changes on storage of its latex, which can be vulcanized in conventional manner, and which possesses in its vulcanized state good mechanical properties.